Custom tick mark in a custom checkobx is not displaying

I have a custom checkbox which I want to add a tick mark inside it when checked

Here is HTML I have:

  <p class="checkbox">
                                <input type="checkbox" class="checkbox  checkbox-custom" name="cgv" id="cgv" value="1" {if $checkedTOS}checked="checked"{/if} />
                                <label class="checkbox-custom-label" for="cgv">{l s='I agree to the terms of service' mod='threepagecheckout'}</label>
                                <a href="{$link_conditions|escape:'html':'UTF-8'}" class="iframe" rel="nofollow">{l s='(Read the Terms of Service)' mod='threepagecheckout'}</a>
                            </p>

Here is css;

input[type="checkbox"] {
    transform: scale(3, 3) !important;
    margin: 0px 21px;
}

.checkbox-custom,
.checkbox-custom-label {
    display: inline-block;
    vertical-align: middle;
    margin: 5px 7px;
    cursor: pointer;
    font-size: 2.4rem;
    font-family: "FuturaPT_BOOK";
    /* padding: 6px; */
}
[type="checkbox"]:not(:checked)+label:after,
[type="checkbox"]:checked+label:after {
  content: '✔';
  position: absolute;
  top: 8px;
  left: 10px;
  font-size: 24px;
  line-height: 0.8;
  color: #fff;
  transition: all .2s;
}

.checkbox-custom+.checkbox-custom-label:before {
    content: '';
    background: #fff;
    border: 1px solid #000;
    display: inline-block;
    vertical-align: middle;
    width: 30px;
    height: 30px;
    padding: 2px;
    margin-right: 30px;
    text-align: center;
    border-radius: 24%;
}

.checkbox-custom:checked+.checkbox-custom-label:before {
    background: #0000;
    box-shadow: inset 0px 0px 0px 4px #fff;
}

unfortunately when I click on check box nothing is displayed, I have tried different combination but nothing worked,

what am I doing wrong?